Typically, the FBI does not discuss or even confirm the existence of ongoing investigations. "But in usual circumstances where it is in the public interest, it may be appropriate to do so".

Testifying before U.S. congress, Mr Comey said he is authorised by the American justice department to make the disclosure. Flynn resigned last month after the leaks revealed that he had privately discussed USA sanctions with the Russian ambassador before Trump took office.

"Mr. President, the Russians hacked our election and interfered".

The committee's ranking member, Rep. "But the Obama administration was committed to the notion, against all evidence, that we could "reset" relations with Putin, and it routinely ignored our warnings".

Rep. Adam Schiff is speaking at the start of the committee's hearing on Russia's interference in the presidential election. He also wants to know if the communications of any campaign officials or associates were subject to any improper surveillance.

"Let me be clear", he said.

Earlier in the hearing, the chairman of the House intelligence Devin Nunes (R-CA) said there was no physical wiretap on Trump Tower, but it's possible that "other surveillance activities" were used against Trump and his associates. Trump has also said he was referring more generally to surveillance, not necessarily bugging of phones.

In his opening statement, Schiff said: "We will never know whether the Russian intervention was determinative in such a close election".

Now that Comey and Rogers have both rejected Trump's allegations that Obama wiretapped him, it remains to be seen whether Trump will withdraw his allegations. More importantly, and for the purposes of our investigation, it simply does not matter. It is no crime for Trump or his associates to have ties to Russian Federation, he added.

Schiff related alleged links between people close to Trump and Russian Federation, including his former campaign aide Paul Manafort, using the President's words during the campaign and publicly available information contained in a dossier drawn up by former British intelligence officer Christopher Steele, large portions of which have yet to be corroborated by CNN. "This is, of course, no crime", Schiff said.

Mr Trump also suggested, without evidence, that Mrs Clinton's campaign was in contact with Russian Federation and had possibly thwarted a federal investigation. The House Intelligence Committee will hear from FBI Director James Comey on whether USA officials believe Russian Federation tried to bolster Trump's chances in the election and if there were any connections between Moscow and Trump's campaign aides. "Must find (the) leaker now!" he said.

On Sunday, Nunes, who served on Trump's transition team, also countered the president's assertion.

Obama did not order a wiretap on Trump Tower. "They're also trying to get involved in campaigns around the globe and over in Europe", he said on "Fox News Sunday". "But there was certainly enough for us to conduct an investigation".

President Trump denied Monday that he colluded with Russian Federation during the presidential election - calling the allegations "made up" by Democrats and "FAKE NEWS".

President Donald Trump won Pennsylvania's 20 electoral vote during the November election. "And it has to be thoughtful, because we are feeding into this covert-action narrative that the Russians are trying to create".

Beyond the hacking and dumping of documents, he said, a key issue is "were there US persons that were helping the Russians in any way?" "Was there any form of collusion?"

Although Trump, as recently as Friday, has continued to defend the extraordinary allegations that the Obama administration wiretapped Trump Tower in advance of the 2016 elections, House and Senate leaders briefed by the Federal Bureau of Investigation director in recent days have flatly denied such surveillance existed in bipartisan joint statements, leaving the White House virtually alone in asserting such claims.

If Trump or his people cooperated with Russia's so-called "active measures", Schiff said, "It would represent one of the most shocking betrayals of democracy in history". Republicans hope Comey will state that there is no evidence of collusion between Trump aides and officials from Moscow, a move that could begin to break up a cloud of Russian intrigue that has stifled the early weeks of the administration.